1. Field of the Invention
The present invention generally relates to semiconductor cleaning apparatus, and more particularly, to a megasonic liquid stream cleaning apparatus for cleaning metal wiring patterns.
2. Discussion of the Related Art
During the manufacture of semiconductor devices there are many stages that require cleaning of a silicon wafer or other microelectric workpiece. This is especially true during what is known as back end of the line processing where the interconnect wiring for the circuits are fabricated. It is important to remove any metal debris created by fabrication or testing steps since metal debris can lead to shorts which will cause yield or reliability problems.
One method for cleaning microelectronic wafers is to use a high pressure vertically directed water liquid stream incident on a rotating wafer below. The wafer is disposed in a plane which is at right angles to the incident liquid stream. The nozzle of the liquid stream is attached to a reciprocating arm so that the entire wafer can be accessed. Often, the high pressure within the water column causes damage to metal patterns on the wafers, particularly when thin metal wiring is subjected to the liquid stream. A mathematical analysis of the forces exerted by the conventional spin-clean device indicates very large horizontal forces in the region of the boundary layer of the liquid stream. These forces are at times strong enough to destroy the metallurgy since typical circuit lines are vulnerable to shear forces.
Another spin-clean device operates at a significantly lower hydrostatic pressure and does not rely on shear forces to obtain effective cleaning of the microelectronic boards or wafers. The high pressure water column is replaced by a low pressure liquid stream with pressures below an atmosphere. At this pressure, the lateral or shear forces are reduced by at least two orders of magnitude compared to the previously described device. Sufficient water flow, however is still present to rinse away loose or loosened debris. The diameter of the liquid stream can be on the order of 6-7 mm. Megasonic energy from a focused transducer in a chamber under low hydrostatic pressure is directed into the liquid stream near the nozzle. Typically, longitudinal waves are used, generated by a shaped piezoelectric transducer. The predominant forces of the liquid stream incident on a workpiece undergoing cleaning are now in the direction of the stagnation column of the liquid stream, i.e., longitudinal or vertical instead of shear or horizontal. The acoustic energy can be as high as several hundred watts/cm.sup.2. This type of acoustic liquid stream has been shown to be effective in a variety of other cleaning applications, including removal of tar, wax and other debris.
An example of such a megasonic wafer cleaning apparatus is described in commonly assigned U.S. Pat. No. 5,368,054 to Koretsky et al., incorporated herein by reference. An example of a commercial system that uses megasonic energy in the cleaning process is Dainippon Screen Manufacturing Company's SP-W813-AS Spin Processor for post CMP cleaning.
However, even with systems that rely on acoustic energy it is often difficult to provide and maintain the right amount of energy to loosen and remove the metal debris without damaging some of the delicate metal wiring lines.